In the manufacture of integrated circuits the various circuit components are defined by using a photoresist layer which is applied to the surface of a semiconductor wafer, exposed and developed. The photoresist is applied to the surface of the wafer by applying a few drops of liquid resist onto a spinning wafer to obtain a photoresist film of predetermined thickness. The photoresist liquid film is prebaked to remove any excess solvent that is initially added to the resist as well as to anneal the polymer film, allowing the resin molecules to relax into their lowest energy state. This thin film of resist is typically on the order of 1-2 .mu.m thick. The film thickness changes about 0.5 .mu.m during prebake.
If the solvent is not fully evaporated or if the prebake time is too long, then the feature size of the developed film may not be as small as expected. It is important when heating the resist for prebake that the temperature of the film reach and exceed the glass transition temperature (T.sub.g) in order to facilitate the removal of solvent and formation of polymer chains. It is important that the bake time be controlled. If the glass transition temperature is measure then the processing time may be easily controlled. In practice, a calibration run will provide the time T, after T.sub.g is reached, required to complete the prebake cycle. It is also important to monitor the postbake or cure process after exposure and before development so that the wafer is not heated above the softening temperature s where features would be altered.
Several techniques are available to measure T.sub.g but none of them have been applied in situ for endpoint detection of photoresist softbake. There has been research in endpoint detection of the prebake process. T. E. Metz et al. "Process Module Metrology Control and Clustering", SPIE, Vol. 1594, 1991, pp. 145-152, performed real-time measurement of resist film thickness on silicon wafers using multi-wavelength reflection interferometry. They determined resist thickness vs. spin and bake time. This method was used to monitor non-uniformities for statistical process control.
There is a need for in situ photoresist monitoring as feature size surpasses sub-quarter micron size since it is expected that the resist materials will be more sensitive to process conditions such as temperature and time.